The present invention relates to composite ceramics and a method of making the same. In particular, the present invention relates to composite ceramics which have a small size change caused by sintering and are suitable for forming parts of complicated shape, and a method of making the above composite ceramics.
Metal and ceramic heaters have hitherto been known, and the metal heater is made of such a material as nichrome or tantalum. However, nichrome and tantalum are low in heat resistance, and deteriorate when heated to 1000.degree. C. or more. The ceramic heater which has been put to practical use, is made of silicon carbide, stabilized zirconia, lanthanum chromide, molybdenum silicide, or the like. These ceramic materials are relatively high in electric resistance, and negative in temperature coefficient of resistance. Accordingly, these materials readily make thermal runaway, and it is difficult to control the temperature of these materials. Further, the ceramic materials are low in mechanical strength and thermal shock resistance. Thus, novel conductive ceramics which can be used in place of the above ceramic materials, have been earnestly desired and widely investigated.
For example, a ceramic body disclosed in JP-A-60-103078, that is, a ceramic body formed in such a manner that a compound is mixed with SiC or Si.sub.3 N.sub.4 and the mixture thus obtained is sintered by the hot press method, can eliminate the difficulties of the conventional ceramic materials. The hot press method can increase the density of sintered body, but requires a vast amount of energy. Thus, there arises a problem that the manufacturing cost the sintered body is increased.
Further, as disclosed in JP-A-60-60,983, a conductive ceramic body can be formed in such a manner that a conductive compound is mixed with Si.sub.3 N.sub.4 and the mixture is sintered by a pressureless sintering method which is less in energy consumption than the hot press method. In this case, however, a sintering agent is added to the above mixture, and thus the sintered body is softened or deformed when heated to a high temperature. Further, the contraction due to sintering lies in a range from about 15% to about 18%, and the deformation due to sintering becomes a serious problem. When the sintering agent is omitted and the mixture of the conductive compound and Si.sub.3 N.sub.4 powder is sintered without containing the sintering agent, the sintered body is low in density and large in resistivity, and hence cannot act as a conductive ceramic body.
As mentioned above, the above prior art pays no attention to the contraction due to sintering. That is, it is difficult to form a ceramic part which has a complicated shape and is readily subjected to deformation due to sintering, by the prior art. Further, a cost necessary for machining the sintered body to a desired shape is not low.
Reference may be made to JP-A-58-88169, JP-A-61-101465 and JP-A-61-146754.